The present invention relates to an automatic fine tuning control for a television receiver. More particularly, it relates to a dual mode fine tuning control for expanding the range of the fine tuning control without compromising the sensitivity thereof.
The ordinary automatic fine tuning control (AFT), also referred to as automatic frequency control (AFC), of a television receiver is responsive to the output of the IF stage in the video processing circuitry of the receiver and is tuned to 45.75 mc, the nominal frequency of the IF picture carrier. The automatic fine tuning control is a high gain frequency sensitive circuit producing a DC voltage proportional to the frequency shift of the output of the IF stage from the desired 45.75 mc. The DC voltage produced by the AFT is applied to the tuning circuits of the tuner to control the fine tuning thereof. Because the automatic fine tuning control cannot distinguish the picture carrier from the adjacent channel sound carrier, which is only 1.5 mc. away from the picture carrier of the channel selected, the normal pull-in range of the AFT is limited to approximately .+-.750 kc.
The problem encountered with such narrow range automatic fine tuning is that often the tuner is detuned substantially from the picture carrier of the channel selected so that a pull-in range of 750 kc. is not sufficient to find the picture carrier. Yet it is not sufficient to merely extend the range of this fine tuning control without having the tuner erroneously lock onto the adjacent channel sound carrier and without sacrificing fine tuning sensitivity. It is therefore necessary, if the range of the automatic fine tuning control is extended, to provide means to distinguish between the picture carrier and the adjacent channel sound carrier. Another problem encountered in attempting to expand the pull in range of AFT is the need to provide means to stop the searching process whenever the picture carrier is located and to hold the level fixed that was obtained in the searching process so that the automatic fine tuning control can operate from this fixed level to hold the tuner on the picture carrier.
It is accordingly an object of the present invention to provide an expanded range AFT that will not lock onto adjacent sound carriers.
Another object of the present invention is to provide a dual mode AFT wherein the AFT operation is defeated during searching and search is stopped and the AFT again enabled when the picture carrier is detected.
A further object is to provide a two stage AFT, the first step acting as coarse tuning to cause the tuner to seek the picture carrier and provide a fixed level to the tuner when the picture carrier is located, the second stage providing fine tuning about the fixed level.
A further object is to provide a digital bi-directional stepped search system for the AFT of a television receiver wherein upon detection of the picture carrier the search is stopped and held at the step obtained.
Another object is providing dual mode AFT having the transition between signal seeking and signal holding operation controlled by a harmonic of detected horizontal sync signals.